1. Field of the Invention
The present invention relates to an electronic display (electro-optical device) formed by fabricating an EL (electroluminescence) element on a substrate. Particularly, the present invention relates to an EL display using a semiconductor element (an element employing a semiconductor thin film), and furthermore to electronic equipment using the EL display as a display portion.
2. Description of the Related Art
In recent years, remarkable progress has been made in a technique for forming TFTs on a substrate, and developing the application of TFTs to an active matrix display device is proceeding. TFTs using a poly-silicon film, in particular, have a higher electric field effect mobility (also referred to as mobility) than that of conventional TFTs using an amorphous silicon film, and hence a high speed operation may be made. Thus, control of pixels, which in the past has been controlled by a driver circuit external to a substrate, can now be made by driver circuits formed on the same substrate as the pixels.
Various merits such as reduction of manufacturing cost, miniaturization of a display device, and increase of yield and throughput can be obtained from such an active matrix display device by forming various circuits and elements on the same substrate.
A research on active matrix EL displays having an EL element as a self-luminous element is being actively carried out. The EL display is also referred to as an organic EL display (OLED) or an organic light emitting diode (OLED).
Unlike a liquid crystal display, the EL display is a self-luminous type. The El element has a structure composed of a pair of electrodes (anode and cathode) and an EL layer, which is usually a laminate structure, sandwiched therebetween. The laminate structure (hole transporting layer, light-emitting layer, electron transporting layer) proposed by Tang, et al. from Eastman Kodak Company can be cited as a typical laminate structure of the EL layer. This laminate structure has an extremely high luminescence efficiency, and therefore at present, most of the EL displays in which research and development are proceeding adopt this laminate structure of the EL layer.
In addition to the above laminate structure, a structure in which the layers are laminated on the anode in the order of a hole injection layer, a hole transporting layer, a light-emitting layer, and an electron transporting layer or in the order of a hole injection layer, a hole transporting layer, a light-emitting layer, an electron transporting layer, and an electron injection layer may be formed. The light-emitting layer may be doped with a fluorescent pigment or the like.
The EL layer is a generic term in the present specification indicating all the layers formed between the cathode and anode. Therefore, the above-mentioned hole injection layer, the hole transporting layer, the light-emitting layer, the electron transporting layer, the electron injection layer, etc. are all included in the EL layer.
A predetermined voltage from the pair of electrodes is applied to the EL layer having the above structure, whereby a re-coupling of carriers in the light-emitting layer occurs to thereby emit light. It is to be noted that throughout the present specification, the emission of light by the EL element is called a drive by the EL element. In addition, a luminescent element formed of the anode, the EL layer, and the cathode is called the EL element in the present specification.
A driving method of the analog system (analog drive) can be cited as a driving method of the EL display. An explanation regarding the analog drive of the EL display will be made with references to FIGS. 18 and 19.
FIG. 18 is a diagram showing the structure of a pixel portion in the EL display having the analog drive. A gate signal line (plurality of gate signal lines G1 to Gy) for inputting a selecting signal from a gate signal line driver circuit is connected to a gate electrode of a switching TFT 1801 of the respective pixels. As to a source region and a drain region of the switching TFT 1801 of the respective pixels, one is connected to a source signal line (also called a data signal line) S1 to Sx for inputting an analog video signal whereas the other is connected to a gate electrode of an EL driving TFT 1804 and a capacitor 1808 of each of the pixels, respectively.
A source region of the EL driving TFT 1804 of each of the pixels is connected to a power supply line (V1 to Vx), and a drain region thereof is connected to an EL element 1806, respectively. An electric potential of the power supply lines (V1 to Vx) is called a power supply electric potential. Each of the power supply lines (V1 to Vx) is connected to the capacitor 1808 of the respective pixels.
The EL element 1806 is composed of an anode, a cathode, and an EL layer sandwiched therebetween. When the anode of the EL element 1806 is connected to either the source region or the drain region of the EL driving TFT 1804, the anode and the cathode of the EL element 1806 become a pixel electrode and an opposing electrode, respectively. Alternatively, if the cathode of the EL element 1806 is connected to either the source region or the drain region of the EL driving TFT 1804, then the anode of the EL element 1806 becomes the opposing electrode whereas the cathode thereof becomes the pixel electrode.
It is to be noted that in the present specification, an electric potential of the opposing electrode is referred to as an opposing electric potential and a power supply for applying the opposing electric potential to the opposing electrode is referred to as an opposing power supply. An EL driver voltage, which is the electric potential difference between an electric potential of the pixel electrode and an electric potential of the opposing electrode, is applied to the EL layer.
FIG. 19 is a timing chart illustrating the EL display shown in FIG. 18 when it is being driven by the analog system. A period from the selection of one gate signal line to the selection of a next different gate signal line is called a 1 line period (L). In addition, a period from the display of one image to the display of the next image corresponds to a 1 frame period (F). In the case of the EL display of FIG. 18, there are “y” number of the gate signal lines and thus a “y” number of line periods (L1 to Ly) are provided in 1 frame period.
Because the number of line periods in 1 frame period increases as resolution becomes higher, driver circuits must be driven at high frequencies.
First of all, the power supply lines (V1 to Vx) are held at a constant power supply electric potential, and the opposing electric potential that is the electric potential of the opposing electrode is also held at a constant electric potential. There is a difference in electric potential between the opposing electric potential and the power supply electric potential to a degree that the EL element can emit light.
A selecting signal from the gate signal line driver circuit is fed to the gate signal line G1 in the first line period (L1). An analog video signal is then sequentially inputted to source signal lines S1 to Sx. All the switching TFTs connected to the gate signal line G1 are turned ON to thereby feed the analog video signal that is inputted to the source signal lines to the gate electrode of the EL driving TFT through the switching TFT.
The amount of current flowing in a channel forming region of the EL driving TFT is controlled by the level (voltage) of the electric potential of the signal inputted to the gate electrode of the EL driving TFT. Accordingly, the electric potential applied to the pixel electrode of the EL element is determined by the level of the electric potential of the analog video signal that is inputted to the gate electrode of the EL driving TFT. The emission of light by the EL element is thus controlled by the electric potential of the analog video signal.
The above described operation is repeated and the first line period (L1) ends upon the completion of inputting the analog video signal to the source signal lines S1 to Sx. Note that a period until the completion of inputting the analog video signal to the source signal lines S1 to Sx and a horizontal retrace period may be combined as one line period. Next, a selecting signal is fed to the gate signal line G2 in the second line period (L2). Similar to the first line period (L1), an analog video signal is sequentially inputted to the source signal lines S1 to Sx.
When the selecting signals have been inputted to all the gate signal lines (G1 to Gy), all the line periods (L1 to Ly) are completed to thereby complete 1 frame period. Display is performed by all the pixels in the 1 frame period to form one image. Note that all the line periods (L1 to Ly) and a vertical retrace period may be combined as one frame period.
Thus, the amount of light emitted by the EL element is controlled by the analog video signal and gray-scale display is therefore performed by this control of the amount of light emitted. This system is a driving system which is referred to as the so-called analog drive method where gray-scale display is performed by the variations of the electric potential of the analog video signal fed to the source signal lines.
The state in which the amount of current supplied to the EL element is controlled by the gate voltage of the EL driving TFT will be explained in detail using FIGS. 20A and 20B.
FIG. 20A is a graph showing a transistor characteristic of the EL driving TFT. A curve line denoted by the reference numeral 401 is referred to as IDS−VGS characteristic (or IDS−VGS curve) where the IDS is a drain current and the VGS is a gate voltage. The amount of current flow to an arbitrary gate voltage can be perceived from this graph.
A region within the dotted line indicated by the reference numeral 402 in the above IDS−VGS characteristic is normally the range for driving the EL element. An enlarge view of the region 402 within the dotted line is shown in FIG. 20B.
In FIG. 20B, a region marked by slanted lines is called a saturated area. This region actually indicates a gate voltage that is near a threshold voltage (VTH) or less. The drain current makes exponential changes to the changes of the gate voltage in this region, and therefore current control is carried out based on the gate voltage using this region.
The analog video signal inputted to the plurality of pixels becomes the gate voltage of the EL driving TFT when the switching TFT is ON. In accordance with the IDS−VGS characteristic shown in FIG. 20A, the drain current to the gate voltage becomes 1 to 1 at this point. In other words, the electric potential of the drain region (EL driver electric potential is ON) is determined in correspondence with the voltage of the analog video signal fed to the gate electrode of the EL driving TFT. Then a predetermined drain current flows to the EL element, whereby the EL element emits light according to the amount of luminescence which corresponds to the amount of drain current.
The amount of luminescing by the EL element is thus controlled by the video signal, and gray-scale display is performed in accordance with this control of the amount of luminescing.
However, the above-mentioned analog drive has a drawback in that it is extremely weak to the characteristic variation of the TFT. For example, let's assume a case where the IDS−VGS characteristic of the switching TFT is different from the switching TFT of an adjacent pixel displaying the same tone.
In this case, the drain current of the respective switching TFTs differ depending on the level of variation, with the result of having different gate voltages applied to the EL driving TFTs of each of the pixels. That is, different currents flow to each of the EL elements resulting in having different amounts of luminescence, and therefore the same gray-scale display cannot be performed.
In addition, even if equivalent gate voltages are applied to the EL driving TFTs of each of the pixels, if there are variations in the IDS−VGS characteristic of the EL driving TFTs, then equivalent drain currents cannot be outputted. As is apparent from the graph of FIG. 20A, the region where the drain current exponentially changes to the changes of the gate voltage is used, and therefore if there is even a slight shift in the IDS−VGS characteristic, a situation occurs where there is a vast difference in the outputted amount of current regardless of the fact that equivalent gate voltages were applied. When such situation occurs, in spite of inputting signals having the same voltage, the amount of luminescence of the EL element is immensely different from that of the adjacent pixel caused by the slight variation of the IDS−VGS characteristic.
In fact, the variation of IDS−VGS characteristic becomes a multiplier effect of both of the variations of the switching TFT and the EL driving TFT, thereby becoming more conditionally severe. Thus, the analog drive is very susceptible to the characteristic variation of the TFT, a point which had become an obstacle in the gray-scale display of conventional active matrix EL displays.